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7991Re: [RADIOSHACKDX394] Re: "Hollow" sound with TWO Antennas?

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  • Tom Holden
    Oct 6, 2013
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      I agree that it is probably multipath with path difference in the range of a few milliseconds. However, there is much going on at the same time with co-channel and possibly adjacent channel interference, with its own multipath. At around 1:08 into the video or at about the same time as the audio switches from host to guest, there seems to be an abrupt change. That may be just coincidental or indicate that the quality of the audio feed to the WBCQ transmitter degraded. Also, it is possible that the broadcaster was deliberately adding some echo or reverb to make the originating booth sound like a big space. Possibly all of those things were going on.
      The description “picket fence” sound applies not to hf but to the multipath phenomenon observed at VHF from a receiver in motion or a reflection in motion (e.g. from aircraft) giving rise to the distortion or background noise varying somewhat similarly to what you would hear driving your car close by a picket fence with the window open. In the VHF case the delay between the primary and reflected path is changing with time, causing rapid alternation of constructive and destructive interference at the station frequency. At a high enough rate of change, it is called “flutter”.
      What does happen with multipath via the ionosphere is large path length differences measured in tens and hundred of miles, ignoring the thousands of miles difference for great circle long-short paths. A 186 mile difference results in a 1 millisecond difference in travel times from transmitter to receiver. That causes the relative phase angle of the fields from the two paths to rotate 360 degrees for every 1 kHz change in frequency. Thus the combined field strength will vary in intensity across the spectrum that supports the two paths with a peak every 1 kHz and a valley in between. This is characteristic of a “comb” filter, i.e., peaks = teeth, valleys = gaps. Perhaps that is what Randy means by “picket fence” as it does give rise to the “hollow” sound. Why it sounds hollow is a little harder to explain, requiring the concept of “group delay” to be introduced.
      The phasing of two antennas that are close together, i.e., spaced very much less than the difference in path lengths, has no effect on the comb filtering due to the path difference, except insofar as the two paths deliver their fields with the same polarisation. However, polarisation rotation is an effect of ionospheric propagation and the amount may be different over the two paths. Thus, one antenna may favour one path over the other and changing the phasing of the antennas or disconnecting one may affect the depth of the comb filter, but not the spacing between minima and maxima.
      Tom ve3meo
      Sent: Wednesday, October 02, 2013 3:26 PM
      Subject: RE: RE: Re: [RADIOSHACKDX394] Re: "Hollow" sound with TWO Antennas?

      Sounds like multi-path to me... I wonder if the signals from the two antennas are arriving out of phase with each other causing the picket fence sound.  disconnecting one antenna or the other would definitely cure the problem...


      Randy, KI6WAS

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